Downy mildew spray program critical for all cucurbit growers

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Fungicides will be needed for the duration of the growing season to protect cucurbit crops from downy mildew. This disease is extremely destructive and can kill an unprotected crop within 7 to 10 days. Since downy mildew is spreading across Michigan, no one should assume that their crop willnot become infected.

There are three elements mandatory for a successful downy mildew program:

1.Use only those fungicides proven to be effective. Data from Michigan State University shows the following program works.

Spray 1: Previcur Flex (1.2 pt.) + Bravo (or Mancozeb)

Spray 2: Tanos 50 DF (8 oz.) + Mancozeb (or Bravo)

All products listed above must be used at full label rates. Cutting rates will leave the crop vulnerable to downy mildew. As a mixing partner, Mancozeb is good against downy mildew but carries a 5-day pre-harvest interval. This makes Mancozeb use especially difficult for growers of hand-harvested cucumber, zucchini and summer squash. Remember, Mancozeb is not registered for use on pumpkins, but Maneb can be used. Some growers are using copper as a mixing partner with Previcur Flex or Tanos 50 DF when the pre-harvest interval prevents the use of Mancozeb. Copper should not be applied under extremely hot conditions or when the weather prevents rapid drying on the leaf surface.

I’ve received a number of questions regarding additional fungicides and their use against downy mildew. I have a large 25-treatment downy mildew trial currently underway. My preliminary observations indicate that Ranman (2.75 fl oz), a relatively new fungicide, can be included in the program of Previcur Flex + Bravo (or Mancozeb) alternated with Tanos 50 DF + Mancozeb (or Bravo) if the downy mildew is not yet present in the field. Last year, when Ranman was included in a field trial where the disease was well established, it did not look good. However, growers of hand-picked cucumbers are in a real bind because of pre-harvest intervals. Previcur Flex has a 2-day pre-harvest interval, Tanos 50 DF has a 3-day pre-harvest interval, and Mancozeb has a 5-day pre-harvest interval. Ranman has a 0-day pre-harvest interval and should be a big help to those needing to get into the field to harvest. However, Ranman must be used prior to downy mildew infection for best results.

2. Keep the spray interval short. Cucurbits grow rapidly and the new growth must be protected. That means a 5-day spray interval for many growers is the only option. Especially growers near the hardest hit areas of the state (see map) must assume that the downy mildew spore load is high (see spore chart) and so is the threat of disease. Those growers with the most severe downy mildew problems have stretched their spray interval and left their crop unprotected at critical times. Remember, that if you use a fungicide in your rotation that is not effective against downy, you also leave your crop vulnerable because the interval between the fungicides that are working becomes too long. The extremely high temperatures of the last several days will limit the spore production of the downy mildew, but the pathogen will rev back up again as soon as the temperatures go back to a seasonal level.

3.Use ground sprayers. Some growers are reporting downy mildew problems in fields that have been sprayed with airplanes. Fungicides for downy mildew control should be applied with ground rigs in a volume of water of at least 30 gals. Using a higher amount of water such as 50 gallons is preferred to ensure uniform and thorough plant coverage.

Table 1. Spore trap daily totals (counts/m 3/day) by location.
Note: currently only the spore trap in Monroe County is in a field with downy mildew disease.

Date

Michigan counties

Allegan

Bay

Monroe

Saginaw

St. Joseph

Van Buren

July 1

0

10

*52

12

7

3

2

2

8

115

8

2

3

3

0

5

8,933

32

0

2

4

2

8

4,345

12

0

15

5

2

2,448

3

0

5

6

2

1,045

8

5

30

7

0

5

*1,557

2

13

2

8

2

8

6,268

8

20

20

9

5

5

*122

3

0

7

10

3

0

*93

5

3

5

11

2

13

3,970

5

0

2

12

2

2

48,878

5

5

13

13

12

23

48,230

7

18

30

14

12

13

2,162

2

3

5

15

3

2

52,127

5

12

30

16

8

2

14,185

0

22

63

17

3

3

7,637

0

7

53

18

2

10

2,410

3

2

37

19

13

37

*62,013

35

90

92

20

3

10

51,098

2

62

210

21

47

3

*53,398

3

72

288

22

108

40

…85

2

287

270

23

28

10

197

10

87

135

24

65

55

510

100

258

618

25

**

**

482

**

297

703

26

**

**

412

**

**

**

27

**

**

502

**

**

**

28

**

**

533

**

**

**

29

**

**

1,660

**

**

**

30

**

**

1,773

**

**

**

*Spores present but field soil prevents accurate count.
**Spore tapes available, counts pending.
…Spore trap moved to new location.
Note: Check my website for updates to this table. (http://www.plantpathology.msu.edu/labs/hausbeck/hausbeck.htm)

Dr. Hausbeck’s work is funded in part by MSU‘s AgBioResearch.

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